The demand for high pressure oxygen gas is increasing due to the greater use of high pressure oxygen in partial oxidation processes such as coal gasification for power generation, hydrogen production, and steelmaking. Often nitrogen is also employed in these processes.
Oxygen gas is produced commercially in large quantities generally by the cryogenic rectification of air. One way of producing the oxygen gas at high pressure is to compress the product oxygen gas from the cryogenic rectification plant. This, however, is costly both in terms of the capital costs for the product oxygen compressor and also in terms of the operating costs to power the product oxygen compressor. Another way of producing high pressure oxygen gas is to operate the cryogenic rectification plant at a higher pressure thus producing the oxygen at a higher initial pressure and reducing or eliminating downstream compression requirements. Unfortunately, operating the cryogenic rectification plant at a higher pressure reduces the efficiency of the production process because component separation depends on the relative volatilities of the components which decrease with increasing pressure. This is particularly the case when high pressure nitrogen product is also desired from the cryogenic rectification plant because the removal of nitrogen from the high pressure distillation column as product reduces the amount of reflux which may be employed thus reducing oxygen recovery.
In response to this problem there have been developed air separation processes wherein liquid oxygen is pressurized, such as by pumping or by hydrostatic means, and vaporized against an air stream which is either partially or totally condensed. This markedly reduces the compression costs for the elevated pressure oxygen gas product.
One problem with such systems is that the condensed air enters the high pressure column of the air separation plant near the bottom of the column. The air undergoes practically no distillation compared to air entering as a vapor at the bottom of the high pressure column. As a result, nitrogen, which is usually available as liquid nitrogen reflux for operation of the high pressure column and the low pressure column when all air enters the high pressure column as a vapor, is not separated from the liquid air. Since the reflux ratio of the high pressure column is fixed by the purity of reflux withdrawn from the top of the column and the number of equilibrium stages present in the column, there is produced less reflux for operation of the upper column resulting in the loss of product.
Nitrogen from the column system may be used in place of feed air to vaporize the liquid oxygen. However such an arrangement often results in the generation of more reflux than needed for the column system thus wasting power. Moreover, if the nitrogen is taken from the lower pressure column, significant power and capital costs are incurred in order to get the nitrogen to the requisite pressure for the product vaporization.
Accordingly, it is an object of this invention to provide a cryogenic rectification system which can produce product gas with improved efficiency over results attainable with conventional systems, especially at elevated product pressure.
It is another object of this invention to provide a cryogenic rectification system which can produce gas with improved efficiency wherein the amount of reflux generated may be adjusted to optimize the system performance.